The invention pertains to fiber optic sensors and, particularly, to fiber optic voltage sensors.
Over the past decade, fiber optic sensors have received attention in the application of magnetic field sensing and current sensing. Fiber optic current sensors are advantageous over iron-core current transformers, since fiber optic sensors are non-conductive and lightweight. Furthermore, fiber optic sensors also do not exhibit hysteresis and provide a much larger dynamic range and frequency response.
Fiber optic current sensors work on the principle of the Faraday effect. Current flowing in a wire induces a magnetic field, which, through the Faraday effect, rotates the plane of polarization of the light traveling in the optical fiber wound around the current carrying wire. Faraday's law, stated as EQU I=H.multidot.dL
where I is the electrical current, H is the magnetic field and the integral is taken over a closed path around the current. If the sensing fiber is wound around the current carrying wire with an integral number of turns, and each point in the sensing fiber has a constant sensitivity to the magnetic field, then the rotation of the plane of polarization of the light in the fiber depends on the current being carried in the wire and is insensitive to all externally generated magnetic fields such as those caused by currents carried in nearby wires. The angle, .DELTA..phi., through which the plane of polarization of light rotates in the presence of a magnetic field is given by EQU .DELTA..phi.=V.intg.H.multidot.dL
where V is the Verdet constant of the fiber glass. The sensing optical fiber performs the line integral of the magnetic field along its path, which is proportional to the current in the wire, when that path closes on itself. Thus, one has .DELTA..phi.=VNI where N is the number of turns of sensing fiber wound around the current carrying wire. The rotation of the state of polarization of the light due to the presence of an electrical current is measured by injecting light with a well defined linear polarization state into the sensing region, and then analyzing the polarization state of the light after it exits the sensing region. Alternatively, .DELTA..phi. represents the excess phase shift encountered by a circularly polarized light wave propagating the sensing fiber.
This technology is related to the in-line optical fiber current sensor as disclosed in U.S. Pat. No. 5,644,397 issued Jul. 1, 1997, to inventor James N. Blake and entitled "Fiber Optic Interferometric Circuit and Magnetic Field Sensor", which is incorporated herein by reference. Optical fiber sensors are also disclosed in U.S. Pat. No. 5,696,858 issued Dec. 9, 1997, to inventor James N. Blake and entitled "Fiber Optics Apparatus and Method for Accurate Current Sensing", which is incorporated herein by reference.
However, a need has arisen for a fiber optic voltage sensor.